Coil spring steel

ABSTRACT

Disclosed herein is a coil spring steel, comprising, by weight, carbon (C): about 0.51% to about 0.57%, silicon (Si): about 1.35% to about 1.45%, manganese (Mn): about 0.95% to about 1.05%, phosphorus (P): from about 0.003% to about 0.015%, sulfur (S): from about 0.003% to about 0.010%, chromium (Cr): from about 0.70% to about 0.90%, copper (Cu): from about 0.30% to about 0.40%, vanadium (V): from about 0.10% to about 0.15%, aluminum (Al): from about 0.010% to about 0.040%, titanium (Ti) from about 0.010% to about 0.033%, molybdenum (Mo): from about 0.05% to about 0.15%, nickel (Ni): from about 0.25% to about 0.35%, and a balance of iron (Fe) and inevitable impurities to form 100%.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2016-0168516, filed Dec. 12, 2016, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to coil spring steel imparted with improved fatigue life and tensile strength by controlling content of silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), copper (Cu), vanadium (V), aluminum (Al), titanium (Ti), and molybdenum (Mo).

Description of Related Art

Recently, high-strength coil springs with a strength of 120K psi have been applied to vehicles manufactured domestically and abroad. For some vehicle models, high-strength coil springs with a strength of as high as 130K are currently used. With the increase in the strength thereof to 110K to 130K psi, coil springs are manufactured to have lower thicknesses or fewer turns in response to the requirement for vehicles to be lightweight. However, such coil springs are more susceptible to corrosion after chipping/decoating. In addition, a reduction in the thickness of coil springs leads to the absence of design tolerance, which in turn leads to insufficient strength, and coil springs having small thicknesses are at risk of fast progression into complete fracture after they undergo a partial fracture.

In order to avoid such problems, a dual-coating process is applied to some corrosion-susceptible areas, but with the consequent side effect of an excessive increase in material (coating) cost, and thus this is not a fundamental solution. Accordingly, increasing the durability of materials by improving strength/corrosion resistance is a significant problem for the automotive industry to solve at present. The recent direction of car research and development toward high performance, high power, and high efficiency requires that parts thereof be of high strength and lightweight. Steel members for suspension, for example, are indispensably provided with high strength and durability but should simultaneously be lightened under the same car weight/corrosion conditions as have been prevalent in the past.

The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing coil spring steel that is improved in fatigue life and tensile strength by controlling content of silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), copper (Cu), vanadium (V), aluminum (Al), titanium (Ti), and molybdenum (Mo).

Various aspects of the present invention are directed to providing a coil spring steel, comprising, by weight, carbon (C): about 0.51% to about 0.57%, silicon (Si): about 1.35% to about 1.45%, manganese (Mn): about 0.95% to about 1.05%, phosphorus (P): from about 0.003% to about 0.015%, sulfur (S): from about 0.003% to about 0.010%, chromium (Cr): from about 0.70% to about 0.90%, copper (Cu): from about 0.30% to about 0.40%, vanadium (V): from about 0.10% to about 0.15%, aluminum (Al): from about 0.010% to about 0.040%, titanium (Ti): from about 0.010% to about 0.033%, molybdenum (Mo): from about 0.05% to about 0.15%, nickel (Ni): from about 0.25% to about 0.35%, and a balance of iron (Fe) and inevitable impurities to form 100%.

In one embodiment, the coil spring steel has a grain size of about 29 μm or less (e.g., about 29 μm, 28 μm, 27 μm, 26 μm, 25 μm, 24 μm, 23 μm, 22 μm, 21 μm, 20 μm, 19 μm, 18 μm, 17 μm, 16 μm, 15 μm, 14 μm, 13 μm, 12 μm, 11 μm, about 10 μm, or less).

In another exemplary embodiment, the coil spring steel is molded into a product that has a total decarburized layer depth of about 50 μm or less (e.g., about 50 μm, 49 μm, 48 μm, 47 μm, 46 μm, 45 μm, 44 μm, 45 μm, 42 μm, 41 μm, 40 μm, 39 μm, 38 μm, 37 μm, 36 μm, 35 μm, 34 μm, 33 μm, 32 μm, 31 μm, 20 μm, or less) and a ferrite decarburization depth of about 1 μm or less (e.g., about 1 μm, 0.9 μm, 0.8 μm, 0.7 μm, 0.6 μm, 0.5 μm, 0.4 μm, 0.3 μm, 0.2 μm, about 0.1 μm, or less).

In another exemplary embodiment, the coil spring steel is molded into a product that has a fatigue life of about 800,000 cycles or greater and a corrosion fatigue life of about 500,000 cycles or greater.

In another exemplary embodiment, the coil spring steel has a tensile strength of about 2150 MPa or greater (e.g., about 2150 MPa, 2160, 2170, 2180, 2190, 2200, 2250, 2300, 2350, 2400, 2500, 2600, 2700, 2800, 2900, about 3000 MPa, or greater).

In another exemplary embodiment, aluminum (Al) is contained in an amount of from about 0.010% to about 0.030% (e.g., about 0.010%, 0.015%, 0.020%, 0.025%, or about 0.030%) to prevent a coarse inclusion from being formed and the coil spring steel is molded into a product that has a fatigue life of about 850,000 cycles or greater and a corrosion fatigue life of about 550,000 cycles or greater.

In another exemplary embodiment, titanium (Ti) is contained in an amount of from about 0.010% to about 0.030% (e.g., about 0.010%, 0.015%, 0.020%, 0.025%, or about 0.030%) to prevent a coarse precipitate from being formed, and the coil spring steel is molded into a product that has a fatigue life of about 850,000 cycles or greater and a corrosion fatigue life of about 550,000 cycles or greater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph in which tensile strength is plotted against silicon (Si) content.

FIG. 2 is a graph in which impact toughness is plotted against silicon (Si) content.

FIG. 3 is a graph in which the general fatigue life of coil spring product is plotted against silicon (Si) content.

FIG. 4 is a graph in which the corrosion fatigue life of coil spring product is plotted against silicon (Si) content.

FIG. 5 is a graph in which the depth of total decarburization is plotted against silicon (Si) content.

FIG. 6 is a graph in which the depth of ferrite decarburization is plotted against silicon (Si) content.

FIG. 7 is a graph in which tensile strength is plotted against manganese (Mn) content.

FIG. 8 is a graph in which impact toughness is plotted against manganese (Mn) content.

FIG. 9 is a graph in which the general fatigue life of coil spring product is plotted against manganese (Mn) content.

FIG. 10 is a graph in which the corrosion fatigue life of coil spring product is plotted against manganese (Mn) content.

FIG. 11 is a graph in which the general fatigue life of coil spring product is plotted against phosphorus (P) content.

FIG. 12 is a graph in which the depth of corrosion cracks is plotted against phosphorus (P) content.

FIG. 13 is a graph in which the corrosion fatigue life of coil spring product is plotted against phosphorus (P) content.

FIG. 14 is a graph in which the general fatigue life of coil spring product is plotted against sulfur (S) content.

FIG. 15 is a graph in which the depth of corrosion cracks is plotted against sulfur (S) content.

FIG. 16 is a graph in which the corrosion fatigue of coil spring product is plotted against sulfur (S) content.

FIG. 17 is a graph in which tensile strength is plotted against chromium (Cr) content.

FIG. 18 is a graph in which impact toughness is plotted against chromium (Cr) content.

FIG. 19 is a graph in which the general fatigue life of coil spring product is plotted against chromium (Cr) content.

FIG. 20 is a graph in which the corrosion fatigue life of coil spring product is plotted against chromium (Cr) content.

FIG. 21 is a graph in which corrosion rates are plotted against copper (Cu) content.

FIG. 22 is a graph in which the depth of corrosion cracks is plotted against copper (Cu) content.

FIG. 23 is a graph in which impact toughness is plotted against copper (Cu) content.

FIG. 24 is a graph in which the general fatigue life of coil spring product is plotted against copper (Cu) content.

FIG. 25 is a graph in which the corrosion fatigue life of coil spring product is plotted against copper (Cu) content.

FIG. 26 is a graph in which tensile strength is plotted against vanadium (V) content.

FIG. 27 is a graph in which impact toughness is plotted against vanadium (V) content.

FIG. 28 is a graph in which the general fatigue life of coil spring product is plotted against vanadium (V) content.

FIG. 29 is a graph in which the corrosion fatigue life of coil spring product is plotted against vanadium (V) content.

FIG. 30 is a graph in which tensile strength is plotted against aluminum (Al) content.

FIG. 31 is a graph in which the general fatigue life of coil spring product is plotted against aluminum (Al) content.

FIG. 32 is a graph in which the corrosion fatigue life of coil spring product is plotted against aluminum (Al) content.

FIG. 33 is a graph in which tensile strength is plotted against titanium (Ti) content.

FIG. 34 is a graph in which the general fatigue life of coil spring product is plotted against titanium (Ti) content.

FIG. 35 is a graph in which the corrosion fatigue life of coil spring product is plotted against titanium (Ti) content.

FIG. 36 is a graph in which tensile strength is plotted against molybdenum (Mo) content.

FIG. 37 is a graph in which impact toughness is plotted against molybdenum (Mo) content.

FIG. 38 is a graph in which the depth of corrosion cracks is plotted against molybdenum (Mo) content.

FIG. 39 is a graph in which the corrosion fatigue life of coil spring product is plotted against molybdenum (Mo) content.

FIG. 40 is a graph showing tensile strengths of a conventional, commercially available coil spring specimen, and coil spring specimens of the Examples and Comparative Examples.

FIG. 41 is a graph showing impact toughness of a conventional, commercially available coil spring specimen, and coil spring specimens of the Examples and Comparative Examples.

FIG. 42 is a graph showing general fatigue lives of a conventional, commercially available coil spring specimen, and coil spring specimens of the Examples and Comparative Examples.

FIG. 43 is a graph showing corrosion fatigue lives of a conventional, commercially available coil spring specimen, and coil spring specimens of the Examples and Comparative Examples.

FIG. 44 shows depths of ferrite decarburization according to silicon (Si) contents on microscopic images.

FIG. 45 shows grain sizes and aluminum inclusions according to aluminum (Al) contents on microscopic images.

FIG. 46 shows grain sizes and titanium precipitates according to titanium (Ti) contents on microscopic images.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

Below, particular embodiments of the present invention will be described in conjunction with the accompanying drawings.

The present invention addresses coil spring steel comprising, by weight, carbon (C): about 0.51% to about 0.57%, silicon (Si): about 1.35% to about 1.45%, manganese (Mn): about 0.95% to about 1.05%, phosphorus (P): from about 0.003% to about 0.015%, sulfur (S): from about 0.003% to about 0.010%, chromium (Cr): from about 0.70% to about 0.90%, copper (Cu): from about 0.30% to about 0.40%, vanadium (V): from about 0.10% to about 0.15%, aluminum (Al): from about 0.010% to about 0.040%, titanium (Ti): from about 0.010% to about 0.033%, molybdenum (Mo): from about 0.05% to about 0.15%, nickel (Ni): from about 0.25% to about 0.35%, and a balance of iron (Fe) and inevitable impurities to form 100%.

Below, the reasons for the numerical limitations of the components in the composition of the steel according to an exemplary embodiment of the present invention will be described. Unless described otherwise, the unit % given in the following description means % by weight.

carbon (C): from about 0.51% to about 0.57% Carbon (C) is the most effective and important element for increasing the strength of steel. Carbon is dissolved upon austenitizing to form a solid solution, and then may undergo martensitic transformation. With the increase of carbon content therein, the steel increases in hardness, but decreases in toughness. Carbon forms a carbide with other elements, such as iron (Fe), chromium (Cr), and vanadium (V), to increase strength and hardness.

Carbon content less than 0.51% decreases tensile strength and fatigue strength. On the other hand, when carbon is used in an amount exceeding 0.57%, the steel exhibits poor toughness, and decreases in processability with the increase in hardness before quenching. Hence, the content of carbon (C) is limited to range of about 0.51% to about 0.57% (e.g., about 0.51%, 0.52%, 0.53%, 0.54%, 0.55%, 0.56%, or about 0.57%).

Silicon (Si): from about 1.35% to about 1.45%

Forming a solid solution in ferrite with iron, silicon (Si) increases hardness and strength, but is an element that is responsible for lowering elongation and impact resistance. It has great chemical affinity for oxygen.

Given a silicon content less than 1.35%, steel has poor tensile strength and fatigue strength. On the other hand, a silicon content greater than 1.45% causes the steel to decrease in fatigue strength because of decarburization and in processability with the increase of hardness before quenching. Thus, silicon (Si) is used in an amount from about 1.35% to about 1.45% (e.g., about 1.35%, 1.36%, 1.37%, 1.38%, 1.39%, 1.40%, 1.41%, 1.42%, 1.43%, 1.44%, or about 1.45%).

Manganese (Mn): from about 0.95% to about 1.05% Manganese (Mn) contributes to enhancing hardenability and strength in steel upon quenching, but may cause quenching cracks, thermal deformation, and a decrease in toughness if used in excess. This element may form a MnS inclusion with sulfur (S).

A silicon content less than 0.95% contributes negligibly to enhancing the hardenability of steel. On the other hand, if silicon is used in an amount greater than 1.05%, the steel decreases in processability and toughness, and deteriorates in fatigue life with the precipitation of excessive MnS. A range of from about 0.95% to about 1.05% (e.g., about 0.95%, 0.96%, 0.97%, 0.98% 0.99%, 1.00%, 1.01%, 1.02%, 1.03%, 1.04%, or about 1.05%) is given as a limitation to the manganese (Mn) content in the steel.

Phosphorus (P): from about 0.003% to about 0.015%

Phosphorus (P), if uniformly dispersed in steel, enhances machinability without problems.

At a phosphorus content less than 0.003%, steel shows poor machinability. On the other hand, a phosphorus content greater than 0.015% decreases the impact resistance of the steel with the consequent increase of notch sensibility, and also promotes tempering brittleness. The content of phosphorus (P) is thus limited to a range of about 0.003% to about 0.015% (e.g., about 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.010%, 0.011%, 0.012%, 0.013%, 0.015%, or about 0.015%).

Sulfur (S): from about 0.003% to about 0.010%

Forming a MnS inclusion with manganese (Mn), sulfur (S) enhances the processability of steel.

At a sulfur content less than 0.003%, the steel has poor processability. On the other hand, a sulfur content greater than 0.010% forms many MnS inclusions that may serve as crack nuclei to decrease fatigue life and corrosion resistance. Hence, the sulfur (S) content is limited to a range of about 0.003% to about 0.010% (e.g., about 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, or about 0.010%).

Chromium (Cr): from about 0.60% to about 0.80%

Chromium (Cr) is dissolved in an austenite phase to enhance hardenability and to suppress softening resistance upon tempering. It is added to impart steel with mechanical properties such as hardenability and strength. Chromium exhibits the effect of preventing high-silicon (Si) steel from being decarburized.

At a chromium content less than 0.60%, steel decreases in strength excessively, thereby being susceptible to permanent deformation. On the other hand, steel with a chromium content exceeding 0.80% decreases in hardness and toughness, thus undergoing cracking. Further, excessive chromium content increases production costs. The chromium (Cr) content preferably falls within a range of about 0.60% to about 0.80% (e.g., about 0.60%, 0.61%, 0.62%, 0.63%, 0.64%, 0.65%, 0.66%, 0.67%, 0.68%, 0.69%, 0.70%, 0.71%, 0.72%, 0.73%, 0.74%, 0.75%, 0.76%, 0.77%, 0.78%, 0.79%, or about 0.80%).

Copper (Cu): from about 0.25% to about 0.35%

Copper (Cu) prevents the progression of inward corrosion of steel as it improves the densification of oxide corrosion scales formed on the surface of the steel. If used excessively, copper incurs red shortness, resulting in the generation of cracks in the steel.

At a copper content less than 0.25%, the steel is susceptible to corrosion and decreases in fatigue life. On the other hand, a copper content greater than 0.35% is apt to induce red shortness, thus cracking the steel, and furthermore, increases production costs. Hence, the copper content is limited to a range of about 0.25% to about 0.35% (e.g., about 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.30%, 0.31%, 0.32%, 0.33%, 0.34%, or about 0.35%).

Vanadium (V): from about 0.05% to about 0.15%

Vanadium (V) is an element that forms microstructural precipitates at high temperatures, thus restraining the migration grain boundaries through microstructural refinement. The microstructural refinement function of vanadium contributes to enhancing the strength and toughness of the steel. However, vanadium, if used in excess, makes precipitates coarse, resulting in decreased toughness and fatigue life.

At a vanadium content of less than 0.05%, the steel decreases in strength and exhibits the migration of grain boundaries. On the other hand, when vanadium is used in an amount exceeding 0.15%, the steel decreases in toughness and fatigue life, and production costs are increased. Thus, the vanadium (V) content is limited to a range of about 0.05% to about 0.15% (e.g., about 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, 0.13%, 0.14%, or about 0.15%).

Aluminum (Al): from about 0.010% to about 0.040%

Aluminum (Al) induces the refinement of austenite and improves strength and impact toughness. Particularly, the addition of Al together with titanium (Ti) and molybdenum (Mo) can reduce the amount of expensive elements, including vanadium for microstructural refinement and nickel for toughness improvement.

An aluminum content less than 0.0010% is not expected to improve strength and impact toughness. When the aluminum content exceeds 0.040%, on the other hand, a coarse inclusion (Al₂O₃) may form, serving as fatigue nuclei to weaken the steel by reducing durability, such as fatigue life. Hence, the aluminum (Al) content preferably falls within a range of about 0.010% to about 0.040% (e.g., about 0.010%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.020%, 0.021%, 0.022%, 0.023%, 0.024%, 0.025%, 0.026%, 0.027%, 0.028%, 0.029%, 0.030% 0.031%, 0.032%, 0.033%, 0.034%, 0.035%, 0.036%, 0.037%, 0.038%, 0.039%, or about 0.040%).

Titanium (Ti): from about 0.010% to about 0.033%

Titanium (Ti) prevents or restrains grain recrystallization and growth. In addition, titanium (Ti) forms nanocarbides such as TiC, TiMoC, etc., thereby increasing strength and fracture toughness. The element reacts with nitrogen to form TiN, which restrains grain growth. Further, it forms TiB₂, which interferes with binding between B and N, with the consequent minimization of the BN-induced quenching property degradation.

A titanium content less than 0.010% is not expected to increase strength and fracture toughness. On the other hand, a titanium content exceeding 0.033% increases production costs, and forms corniculate precipitates to decrease fatigue life. A range of about 0.010% to about 0.033% (e.g., about 0.010%, 0.011%, 0.012%, 0.013%, 0.014%, 0.015%, 0.016%, 0.017%, 0.018%, 0.019%, 0.020%, 0.021%, 0.022%, 0.023%, 0.024%, 0.025%, 0.026%, 0.027%, 0.028%, 0.029%, 0.030% 0.031%, 0.032%, or about 0.033%) is thus given to a titanium content.

Molybdenum (Mo): from about 0.05% to about 0.15%

Molybdenum (Mo) forms microstructural carbide precipitates such as TiMoC to improve strength and fracture toughness.

When used in an amount less than 0.05%, molybdenum cannot form carbides, failing to acquire sufficient strength and fracture toughness. On the other hand, a molybdenum content exceeding 0.15% is disadvantageous in terms of processability and thus productivity. A preferable molybdenum (Mo) content falls within a range of about 0.1% to about 0.5% (e.g., about 0.10%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.30% 0.31%, 0.32%, 0.33%, 0.34%, 0.35%, 0.36%, 0.37%, 0.038%, 0.39%, 0.40%, 0.41%, 0.42%, 0.43%, 0.44%, 0.45%, 0.46%, 0.47%, 0.48%, 0.49%, or about 0.50%).

Nickel (Ni): from about 0.25% to about 0.35%

Nickel (Ni) is an element that is used to strengthen the matrix of steel since it functions to make steel structurally fine and forms a solid solution in an austenite phase. It provides high hardenability and improved corrosion resistance for steel.

At a nickel content less than 0.25%, the steel is apt to corrode and decrease in fatigue life due to poor corrosion resistance. A nickel content exceeding 0.35% increases costs. Thus, the nickel (Ni) content is limited to the range of about 0.25% to about 0.35% (e.g., about 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.30% 0.31%, 0.32%, 0.33%, 0.34%, or about 0.35%).

EXAMPLES AND COMPARATIVE EXAMPLES

Effects attributed to the control of silicon (Si) content as in Table 1 are delineated in FIGS. 1 to 6.

TABLE 1 C Si Mn P S Cr Cu V Al Ti Mo Ni C. Ex. 1 0.54 1.14 1.01 0.008 0.008 0.80 0.32 0.12 0.020 0.020 0.110 0.290 C. Ex. 2 0.55 1.27 1.02 0.007 0.009 0.80 0.31 0.12 0.021 0.021 0.100 0.280 Ex. 1 0.54 1.35 1.00 0.009 0.010 0.81 0.31 0.11 0.019 0.022 0.110 0.280 Ex. 2 0.56 1.45 1.01 0.008 0.009 0.79 0.32 0.12 0.019 0.021 0.100 0.270 Ex. 3 0.54 1.53 1.02 0.007 0.008 0.79 0.30 0.10 0.021 0.020 0.100 0.290

In the Examples and Comparative Examples of Table 1, the content of silicon (Si) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the silicon (Si) content is limited to the range of 1.35% to 1.45% in the present invention, Comparative Examples 1 and 2 had silicon content less than 1.35% while Comparative Example 3 had silicon content greater than 1.45%.

With the increase in silicon content, as can be seen in FIG. 1, FIG. 2, and FIG. 3, tensile strength increased and the spring steel product increased in fatigue life. However, impact toughness, as shown in FIG. 2, decreased with the increase of silicon content, and rapidly decreased in the silicon content range of 1.45% to 1.53%.

For the measurement of tensile strength, a standard specimen was used according to KS B 0801 while impact toughness was measured according to KS D ISO 148-1.

Coil spring steel products were measured for fatigue life using a spring fatigue tester under a repeated stress of 20 to 120 kgf/mm3.

As is understood from the data of FIG. 4, a silicon (Si) content range of 1.35% to 1.45% was optimal with regard to the corrosion fatigue life of the spring product. In the silicon content range (1.45% to 1.53%), where impact toughness rapidly decreases due to a notch effect on corrosion cracks, the corrosion fatigue life of the spring product also decreases.

Coil spring steel products were measured for corrosion fatigue life using a spring fatigue tester under a repeated stress of 20 to 60 kgf/mm3 while a 5% aqueous NaCl solution was sprayed at 35° C.

As shown in FIG. 5, the steel maintained a total decarburized depth of 40 to 50 μm at a Si content of 1.35% to 1.45%, but the total decarburized layer depth rapidly increased when the Si content ranged from 1.45% to 1.53%. The term “total decarburized layer depth” refers to a depth to which a coil spring steel decreases in hardness as its carbon is lost by heat treatment. A greater total decarburized layer depth acts as a more serious factor to reduce a coil spring steel product in fatigue life and corrosion fatigue life.

A total decarburized layer depth is measured using a hardness method. It is defined as the depth from a surface to a point at which hardness starts to rapidly increase.

Turning to FIG. 6, the depth of ferrite decarburization is maintained at a level of 1 μm or less with a silicon content up to 1.45%, and rapidly increases within a silicon content range of 1.45% to 1.53%. The depth of ferrite decarburization means a white ferrite structure that appears when large carbon loss occurs on the surface of a coil spring steel. A depth of ferrite decarburization of up to 1 μm does not significantly affect general fatigue life or corrosion fatigue life, but when exceeding 1 μm, the depth of ferrite decarburization acts as a factor to reduce coil spring steel products in general fatigue life and corrosion fatigue life, like the total decarburized layer depth.

The depth of ferrite decarburization is measured using microscopy. The depth of a white ferrite structure can be determined from a microscopic cross-sectional image. As seen in FIG. 44, a white ferrite structure cannot be definitely observed when the depth of white ferrite decarburization was formed less than 1 μm.

Accordingly, it is reasonable to limit the silicon (S) content to a range of 1.35% to 1.45%.

Effects attributed to the control of manganese (Mn) content as in Table 2 are delineated in FIGS. 7 to 10.

TABLE 2 C Si Mn P S Cr Cu V Al Ti Mo Ni C. Ex. 4 0.54 1.41 0.82 0.008 0.008 0.80 0.32 0.12 0.020 0.020 0.110 0.290 C. Ex. 5 0.55 1.39 0.87 0.007 0.009 0.80 0.31 0.12 0.021 0.021 0.100 0.280 Ex. 3 0.54 1.39 0.95 0.009 0.010 0.81 0.31 0.11 0.019 0.022 0.110 0.280 Ex. 4 0.56 1.40 1.05 0.008 0.009 0.79 0.32 0.12 0.019 0.021 0.100 0.270 Ex. 6 0.54 1.41 1.17 0.007 0.008 0.79 0.30 0.10 0.021 0.020 0.100 0.290

In the Examples and Comparative Examples of Table 2, the content of manganese (Mn) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the manganese (Mn) content is limited to the range of 0.95% to 1.05% in the present invention, Comparative Examples 4 and 5 had manganese (Mn) content less than 0.95% while Comparative Example 6 had manganese (Mn) content greater than 1.05%.

With the increase in manganese (Mn) content, as can be seen in FIGS. 7 to 9, tensile strength increased and the spring steel product increased in fatigue life. However, impact toughness, as shown in FIG. 8, tended to decrease with the increase of manganese (Mn) content, and rapidly decreased in the manganese content range of 1.05% to 1.17%.

As is understood from the data of FIG. 10, a manganese (Mn) content range of 0.95˜1.05% was optimal with regard to the corrosion fatigue life of the spring product. In the manganese content range (1.05% to 1.17%), in which impact toughness rapidly decreases due to a notch effect on corrosion cracks, the corrosion fatigue life of the spring product also decreases.

Meanwhile, the depths of both total decarburization and ferrite decarburization were almost completely unaffected by the content of manganese (Mn).

Effects attributed to the control of phosphorus (P) content as in Table 3 are delineated in FIGS. 11 to 13.

TABLE 3 C Si Mn P S Cr Cu V Al Ti Mo Ni Ex. 5 0.54 1.39 1.01 0.003 0.008 0.80 0.32 0.12 0.020 0.020 0.110 0.290 Ex. 6 0.55 1.41 1.02 0.011 0.009 0.80 0.31 0.12 0.021 0.021 0.100 0.280 Ex. 7 0.54 1.40 1.00 0.015 0.010 0.81 0.31 0.11 0.019 0.022 0.110 0.280 C. Ex. 7 0.56 1.40 1.01 0.021 0.009 0.79 0.32 0.12 0.019 0.021 0.100 0.270 C. Ex. 8 0.54 1.41 1.02 0.030 0.008 0.79 0.30 0.10 0.021 0.020 0.100 0.290

In the Examples and Comparative Examples of Table 3, the content of phosphorus (P) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the phosphorus (P) content is limited to the range of 0.003% to 0.015% in the present invention, Comparative Examples 7 and 8 had phosphorus (P) content greater than 0.015%.

As shown in FIG. 11, the coil spring steel products had a general fatigue life of around 800,000 cycles even though the phosphorus (P) content thereof increased, indicating that the control of phosphorus (P) content had no significant influence on the general fatigue life of coil spring steel products.

On the other hand, as is understood from data of FIG. 12, and FIG. 13, a higher phosphorus (P) content resulted in deeper corrosion cracking and a shorter corrosion fatigue life in the coil spring steel product. Further, the corrosion crack rapidly deepened and the corrosion fatigue life of the coil spring steel product was rapidly reduced within the P content range of 0.015% to 0.021%. These results suggest that a phosphorus (P) content exceeding the upper limit brings about a reduction in impact resistance and promotes tempering brittleness.

Corrosion resistance is measured in terms of the depth (μm) of corrosion cracks as the specimen is treated at 35° C. for 360 hrs while being sprayed with a 5% NaCl solution. Shallower corrosion cracks represent better corrosion resistance.

Accordingly, it is reasonable to limit the phosphorus (P) content to a range of 0.003% to 0.015%.

Effects attributed to the control of sulfur (S) content as in Table 4 are delineated in FIGS. 14 to 16.

TABLE 4 C Si Mn P S Cr Cu V Al Ti Mo Ni Ex. 8 0.54 1.39 1.01 0.008 0.003 0.80 0.32 0.12 0.020 0.020 0.110 0.290 Ex. 9 0.55 1.39 1.02 0.007 0.005 0.80 0.31 0.12 0.021 0.021 0.100 0.280 Ex. 10 0.54 1.38 1.00 0.009 0.010 0.81 0.31 0.11 0.019 0.022 0.110 0.280 C. Ex. 9 0.56 1.41 1.01 0.008 0.021 0.79 0.32 0.12 0.019 0.021 0.100 0.270 C. Ex. 10 0.54 1.40 1.02 0.029 0.008 0.79 0.30 0.10 0.021 0.020 0.100 0.290

In the Examples and Comparative Examples of Table 4, the content of sulfur (S) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the sulfur (S) content is limited to the range of 0.003 to 0.010% in the present invention, Comparative Examples 9 and 10 had sulfur (S) content greater than 0.010%.

As shown in FIG. 14, the coil spring steel products had a general fatigue life of around 800,000 cycles or higher even though the sulfur (S) content thereof increased, and then rapidly reduced in general fatigue life within the sulfur (S) range of 0.010% to 0.021%, indicating that MnS inclusions have significant influences on general fatigue life as the sulfur (S) content exceeds the upper limit.

In addition, as is understood from the data of FIGS. 15 and 16, a higher sulfur (S) content resulted in deeper corrosion cracking and a shorter corrosion fatigue life in the coil spring steel product. Further, corrosion cracks rapidly deepened and the corrosion fatigue life of the coil spring steel product was rapidly reduced within the sulfur (S) content range of 0.010% to 0.021%. These results suggest that a phosphorus (P) content exceeding the upper limit renders MnS inclusions that serve to initiate corrosion.

Accordingly, it is reasonable to limit the sulfur (S) content to a range of 0.003 to 0.010%.

Effects attributed to the control of chromium (Cr) content as in Table 5 are delineated in FIGS. 17 to 20.

TABLE 5 C Si Mn P S Cr Cu V Al Ti Mo Ni C. Ex. 11 0.54 1.49 1.01 0.008 0.008 0.65 0.32 0.12 0.020 0.020 0.110 0.290 Ex. 11 0.55 1.50 1.02 0.007 0.009 0.70 0.31 0.12 0.021 0.021 0.100 0.280 Ex. 12 0.54 1.51 1.00 0.009 0.010 0.81 0.31 0.11 0.019 0.022 0.110 0.280 Ex. 13 0.56 1.51 1.01 0.008 0.009 0.90 0.32 0.12 0.019 0.021 0.100 0.270 C. Ex. 12 0.54 1.50 1.02 0.007 0.008 0.94 0.30 0.10 0.021 0.020 0.100 0.290

In the Examples and Comparative Examples of Table 5, the content of chromium (Cr) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the chromium (Cr) content is limited to the range of 0.70% to 0.90% in the present invention, Comparative Example 11 had chromium (Cr) content less than 0.70% while Comparative Example 12 had chromium (Cr) content greater than 0.90%.

With the increase of chromium (Cr) content, as can be seen in FIGS. 17 and 19, the spring steel product increased in tensile strength and general fatigue life. However, impact toughness, as shown in FIG. 18, decreased with the increase of chromium (Cr) content, and rapidly decreased in the silicon content range of 0.90% to 0.94%.

As is understood from the data of FIG. 20, a chromium (Cr) content range of 0.70% to 0.90% was optimal for the corrosion fatigue life of the spring product. In the chromium (Cr) content range (0.90% to 0.94%) where impact toughness rapidly decreases due to a notch effect on corrosion cracks, the corrosion fatigue life of the spring product also decreased.

Effects attributed to the control of copper (Cu) content as in Table 6 are delineated in FIGS. 21 to 25.

TABLE 6 C Si Mn P S Cr Cu V Al Ti Mo Ni C. Ex. 13 0.54 1.51 1.01 0.008 0.008 0.80 0.28 0.12 0.020 0.020 0.110 0.290 Ex. 14 0.55 1.49 1.02 0.007 0.009 0.80 0.30 0.12 0.021 0.021 0.100 0.280 Ex. 15 0.54 1.49 1.00 0.009 0.010 0.81 0.36 0.11 0.019 0.022 0.110 0.280 Ex. 16 0.56 1.50 1.01 0.008 0.009 0.79 0.40 0.12 0.019 0.021 0.100 0.270 C. Ex. 14 0.54 1.51 1.02 0.007 0.008 0.79 0.43 0.10 0.021 0.020 0.100 0.290

In the Examples and Comparative Examples of Table 6, the content of copper (Cu) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the copper (Cu) content is limited to the range of 0.30% to 0.40% in the present invention, Comparative Example 13 had copper (Cu) content less than 0.30% while Comparative Example 14 had copper (Cu) content greater than 0.40%.

As can be seen in FIGS. 21 and 22, a lower corrosion rate and shallower corrosion cracking were observed at a higher copper (Cu) content.

The corrosion rate (A/cm2), considered an evaluation index for corrosion resistance, is determined by measuring a current density at 35° C. in a 5% NaCl solution in which a specimen is immersed. A lower current density represents better corrosion resistance.

With the increase of copper (Cu) content, finer oxide corrosion scales are formed on the outermost surface, thus delaying the inward progression of corrosion into the steel. Hence, a high copper (Cu) content acts as a factor to increase corrosion fatigue life.

As shown in FIG. 23, impact toughness tends to decrease with the increase of copper (Cu) content, and is rapidly reduced in the copper (Cu) content range of 0.40% to 0.43%.

There were no significant differences in general fatigue life over the coil spring steel products, as shown in FIG. 24, even though copper (Cu) content increased.

FIG. 25 shows that a copper (Cu) content range of 0.30% to 0.40% was optimal for the corrosion fatigue life of the spring product. When the copper (Cu) content exceeds a critical point, a copper-saturated layer is formed on the surface, increasing brittleness with the consequent rapid reduction of the coil spring product in corrosion fatigue life.

Accordingly, it is reasonable to limit the copper (Cu) content to a range of 0.30% to 0.40%.

Effects attributed to the control of vanadium (V) content as in Table 7 are delineated in FIGS. 26 to 29.

TABLE 7 C Si Mn P S Cr Cu V Al Ti Mo Ni C. Ex. 15 0.54 1.51 1.01 0.008 0.008 0.80 0.32 0.08 0.020 0.020 0.110 0.290 Ex. 17 0.55 1.50 1.02 0.007 0.009 0.80 0.31 0.10 0.021 0.021 0.100 0.280 Ex. 18 0.54 1.50 1.00 0.009 0.010 0.81 0.31 0.12 0.019 0.022 0.110 0.280 Ex. 19 0.56 1.51 1.01 0.008 0.009 0.79 0.32 0.15 0.019 0.021 0.100 0.270 C. Ex. 16 0.54 1.49 1.02 0.007 0.008 0.79 0.30 0.17 0.021 0.020 0.100 0.290

In the Examples and Comparative Examples of Table 7, the content of vanadium (V) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the vanadium (V) content is limited to the range of 0.10% to 0.15% in the present invention, Comparative Example 15 had vanadium (V) content less than 0.10% while Comparative Example 16 had vanadium (V) content greater than 0.15%.

With the increase in vanadium (V) content, as can be seen in FIGS. 26 and 28, tensile strength increased and the spring steel product increased in fatigue life. However, impact toughness, as shown in FIG. 27, tended to decrease with the increase of vanadium (V) content, and rapidly decreased in the vanadium content range of 0.15% to 0.17%.

FIG. 29 shows that a vanadium (V) content range of 0.10% to 0.15% was optimal for the corrosion fatigue life of the spring product. In the vanadium content range where both brittleness and impact toughness are reduced due to coarse precipitates and crack sensibility, respectively, the corrosion fatigue life of the spring product is also decreased.

Accordingly, it is reasonable to limit the vanadium (V) content to a range of 0.10% to 0.15%.

Effects attributed to the control of aluminum (Al) content as in Table 8 are delineated in FIGS. 30 to 32.

TABLE 8 C Si Mn P S Cr Cu V Al Ti Mo Ni C. Ex. 17 0.54 1.49 1.01 0.008 0.008 0.80 0.32 0.12 0.001 0.020 0.110 0.290 C. Ex. 18 0.55 1.50 1.02 0.007 0.009 0.80 0.31 0.12 0.005 0.021 0.100 0.280 Ex. 20 0.54 1.51 1.00 0.009 0.010 0.81 0.31 0.11 0.010 0.022 0.110 0.280 Ex. 21 0.56 1.51 1.01 0.008 0.009 0.79 0.32 0.12 0.030 0.021 0.100 0.270 Ex. 22 0.54 1.50 1.02 0.007 0.008 0.79 0.30 0.10 0.040 0.020 0.100 0.290

In the Examples and Comparative Examples of Table 8, the content of aluminum (Al) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the aluminum (Al) content is limited to the range of 0.010˜0.040% in the present invention, Comparative Examples 17 and 18 had aluminum (Al) content less than 0.010% while Example 22 had aluminum (Al) content greater than 0.030%.

FIG. 30 shows that steel underwent no significant changes in tensile strength with the change of aluminum (Al) content.

As can be seen in FIGS. 31 and 32, an aluminum (Al) content range of 0.010˜0.030% was optimal with regard to the general fatigue life and corrosion fatigue life of the spring product. Aluminum (Al) is used to improve fatigue life through microstructural refinement. As aluminum (Al) content increased, both the general fatigue life and the corrosion fatigue life of the coil spring steel products increased to some point and then rapidly decreased.

The increase of both general fatigue life and corrosion fatigue life in the coil spring steel products is attributed to the grain refinement effect of aluminum (Al). After reaching respective maximum points, both general fatigue life and corrosion fatigue life started to decrease since aluminum (Al) inclusions became coarse.

As is understood from the images of FIG. 45, grain sizes were reduced with the increase of aluminum (Al) content, but inclusions were formed at an Al content exceeding a critical point.

Grain sizes were measured to be 36 μm in Comparative Example 17 and 33 μm in Comparative Example 18, and reduced to 27 μm in Example 20 and 24 μm in Example 21, but increased to 25 μm in Example 22. Accordingly, the coil spring steel products rapidly decreased in general fatigue life and corrosion fatigue life over the aluminum (Al) content range of 0.030% to 0.040%.

Accordingly, it is reasonable to limit the aluminum (Al) content to a range of 0.010% to 0.040%, and preferably to a range of 0.010% to 0.030%.

Effects attributed to the control of titanium (Ti) content as in Table 9 are delineated in FIGS. 37 to 35.

TABLE 9 C Si Mn P S Cr Cu V Al Ti Mo Ni C. Ex. 19 0.54 1.51 1.01 0.008 0.008 0.80 0.32 0.12 0.020 0.003 0.110 0.290 C. Ex. 20 0.55 1.49 1.02 0.007 0.009 0.80 0.31 0.12 0.021 0.005 0.100 0.280 Ex. 23 0.54 1.49 1.00 0.009 0.010 0.81 0.31 0.11 0.019 0.010 0.110 0.280 Ex. 24 0.56 1.50 1.01 0.008 0.009 0.79 0.32 0.12 0.019 0.030 0.100 0.270 Ex. 25 0.54 1.51 1.02 0.007 0.008 0.79 0.30 0.10 0.021 0.033 0.100 0.290

In the Examples and Comparative Examples of Table 9, the content of titanium (Ti) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the titanium (Ti) content is limited to the range of 0.010% to 0.030% in the present invention, Comparative Examples 19 and 20 had titanium (Ti) content less than 0.010% while Example 25 had titanium (Ti) content greater than 0.030%.

FIG. 33 shows that steel underwent no significant changes in tensile strength with the change of titanium (Ti) content.

As can be seen in FIGS. 34 and 35, a titanium (Ti) content range of 0.010% to 0.030% was optimal with regard to the corrosion fatigue life and corrosion fatigue life of the spring product. Titanium (Ti) is used to improve fatigue life and maximize the grain-refining effect of boron through microstructural refinement. As titanium (Ti) content increased, both the general fatigue life and the corrosion fatigue life of the coil spring steel products increased to some point and then rapidly decreased.

The increase of both general fatigue life and corrosion fatigue life in the coil spring steel products is attributed to the grain refinement effect of titanium (Ti). After reaching respective maximum points, both general fatigue life and corrosion fatigue life started to decrease since precipitates such as TiN of high hardness, became coarse, serving as fatigue nuclei.

As is understood from the images of FIG. 46, grain sizes were reduced with the increase of titanium (Ti) content, but precipitates were formed at a Ti content exceeding a critical point.

Grain sizes were measured to be 38 μm in Comparative Example 19 and 35 μm in Comparative Example 20, and they were reduced to 29 μm in Example 23 and 25 μm in Example 24, but increased to 26 μm in Example 25. Accordingly, the coil spring steel products rapidly decreased in general fatigue life and corrosion fatigue life over the titanium (Ti) content range of 0.030% to 0.033%.

Accordingly, it is reasonable to limit the titanium (Ti) content to a range of 0.010% to 0.033% and preferably to a range of 0.010% to 0.030%.

Effects attributed to the control of molybdenum (Mo) content as in Table 10 are delineated in FIGS. 36 to 39.

TABLE 10 C Si Mn P S Cr Cu V Al Ti Mo Ni C. Ex. 21 0.54 1.51 1.01 0.008 0.008 0.80 0.32 0.12 0.020 0.020 0.01 0.290 C. Ex. 22 0.55 1.50 1.02 0.007 0.009 0.80 0.31 0.12 0.021 0.021 0.03 0.280 Ex. 26 0.54 1.50 1.00 0.009 0.010 0.81 0.31 0.11 0.019 0.022 0.05 0.280 Ex. 27 0.56 1.51 1.01 0.008 0.009 0.79 0.32 0.12 0.019 0.021 0.10 0.270 Ex. 28 0.54 1.49 1.02 0.007 0.008 0.79 0.30 0.10 0.021 0.020 0.15 0.290 C. Ex. 23 0.54 1.50 1.01 0.008 0.008 0.80 0.32 0.12 0.020 0.020 0.17 0.290 C. Ex. 24 0.54 1.51 1.00 0.009 0.010 0.81 0.31 0.11 0.019 0.022 0.21 0.290

In the Examples and Comparative Examples of Table 10, the content of molybdenum (Mo) alone was used as a variable while the other elements were confined within the respective ranges described according to an exemplary embodiment of the present invention.

Because the molybdenum (Mo) content is limited to the range of 0.05% to 0.15% in the present invention, Comparative Examples 23 and 24 had molybdenum (Mo) content less than 0.05% while Comparative Examples 23 and 24 had molybdenum (Mo) content greater than 0.15%.

With the increase in molybdenum (Mo) content, as can be seen in FIG. 36, tensile strength increased. However, impact toughness, as shown in FIG. 37, tended to decrease with the increase of molybdenum (Mo) content, and rapidly decreased in the molybdenum (Mo) content range of 0.15% to 0.17%.

As is understood from data of FIG. 38, a higher molybdenum (Mo) content resulted in deeper corrosion cracking. In FIG. 39, the coil spring steel product was observed to rapidly decrease in corrosion fatigue life with the increase of molybdenum (Mo) content, but the increase rate slowed or remained unchanged with the molybdenum (Mo) content exceeding 0.15%. The rapid increase of corrosion fatigue life was attributed to an improved strength and a reduced depth of corrosion cracks whereas reduced impact toughness, together with a reduced depth of corrosion cracks, is responsible for the slow or unchanged rate of increase.

In Table 11 and FIGS. 40 to 43, the coil spring steel according to an exemplary embodiment of the present invention are illustrated to have properties superior to those of a conventional commercially available material, or coil spring steel in which any one of silicon (Si), manganese (Mn), phosphorus (P), sulfur (S) chromium (Cr), copper (Cu), vanadium (V), aluminum (Al), titanium (Ti) and molybdenum (Mo) was used in an amount less or higher than the content defined in the present.

TABLE 11 C Si Mn P S Cr Cu V Al Ti Mo Ni Conventional #1 0.54 1.48 0.64 0.01 0.01 0.67 0.28 0.11 0.002 0.035 — 0.28 C. Ex. 25 #2 0.48 1.32 0.92 0.004 0.004 0.68 0.26 0.07 0.008 0.008 0.03 0.22 Ex. 29 #3 0.52 1.37 0.96 0.004 0.004 0.72 0.3 0.1 0.011 0.011 0.06 0.26 Ex. 30 #4 0.55 1.41 0.99 0.009 0.006 0.81 0.34 0.11 0.02 0.021  0.011 0.3 Ex. 31 #5 0.57 1.44 1.03 0.014 0.009 0.88 0.39 0.15 0.029 0.028 0.14 0.34 C. Ex. 26 #6 0.6 1.48 1.08 0.018 0.015 0.94 0.43 0.17 0.033 0.033 0.19 0.37

As shown in FIGS. 40 and 41, the coil spring steel of the present invention was imparted with impact toughness similar to that of the conventional commercially available material, but its tensile strength of more than 2150 MPa can allow for the production of coil spring steel products with a weight of 3 kg or less, contributing to the weight reduction of car.

The coil spring steel products of the present invention, as shown in FIGS. 42 and 43, exhibited a fatigue life of 800,000 cycles or greater and a corrosion fatigue life of 500,000 cycles or greater.

Unlike conventional commercially available members employing a urethane hose as a corrosion resistant reinforcement, the coil spring steel according to an exemplary embodiment of the present invention does not need a urethane hose due to the improved corrosion resistance thereof, which leads to a decrease in production cost.

(Manufacture)

Steel comprising, by weight, carbon (C): about 0.51% to about 0.57%, silicon (Si): about 1.35% to about 1.45%, manganese (Mn): about 0.95% to about 1.05%, phosphorus (P): from about 0.003% to about 0.015%, sulfur (S): from about 0.003% to about 0.010%, chromium (Cr): from about 0.70% to about 0.90%, copper (Cu): from about 0.30% to about 0.40%, vanadium (V): from about 0.10% to about 0.15%, aluminum (Al): from about 0.010% to about 0.040%, titanium (Ti) from about 0.010% to about 0.033%, molybdenum (Mo): from about 0.05% to about 0.15%, nickel (Ni): from about 0.25% to about 0.35%, and a balance of iron (Fe) and inevitable impurities to form 100% was subjected to wire-rod and peeling processes.

Thereafter, the wire rod was maintained at a predetermined temperature, and then air cooled to refine grains, followed by a controlled heat treatment process for structural homogenization. In the controlled heat treatment process, the rod was maintained at 950 to 1000° C. for 4 to 6 min in order to minimize the reduction of hardness in the outermost surface. Subsequently, the homogenized rod was provided with strength and toughness by conducting quenching and tempering processes. As a result, a coil spring product was manufactured.

As described hitherto, the coil spring steel of the present invention is imparted with improved fatigue life and tensile strength through controlling contents of silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), copper (Cu), vanadium (V), aluminum (Al), titanium (Ti), and molybdenum (Mo), and its weight can be reduced correspondingly, thereby contributing to the weight reduction of car.

The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. 

What is claimed is:
 1. A coil spring steel, comprising, by weight, carbon (C): from about 0.51% to about 0.57%, silicon (Si): from about 1.35% to about 1.45%, manganese (Mn): from about 0.95% to about 1.05%, phosphorus (P): from about 0.003% to about 0.015%, sulfur (S): from about 0.003% to about 0.010%, chromium (Cr): from about 0.70% to about 0.90%, copper (Cu): from about 0.30% to about 0.40%, vanadium (V): from about 0.10% to about 0.15%, aluminum (Al): from about 0.010% to about 0.040%, titanium (Ti): from about 0.010% to about 0.033%, molybdenum (Mo): from about 0.05% to about 0.15%, nickel (Ni): from about 0.25% to about 0.35%, and a balance of iron (Fe) and inevitable impurities to form 100%.
 2. The coil spring steel of claim 1, having a grain size of about 29 μm or less.
 3. The coil spring steel of claim 1, wherein the coil spring steel is molded into a product that has a total decarburized layer depth of about 50 μm or less and a ferrite decarburization depth of 1 μm or less.
 4. The coil spring steel of claim 1, wherein the coil spring steel is molded into a product that has a fatigue life of about 800,000 cycles or greater and a corrosion fatigue life of about 500,000 cycles or greater.
 5. The coil spring steel of claim 1, having a tensile strength of about 2150 MPa or greater.
 6. The coil spring steel of claim 2, wherein aluminum (Al) is contained in an amount of from about 0.010% to about 0.030% to prevent a coarse inclusion from being formed and the coil spring steel is molded into a product that has a fatigue life of about 850,000 cycles or greater and a corrosion fatigue life of about 550,000 cycles or greater.
 7. The coil spring steel of claim 2, wherein titanium (Ti) is contained in an amount of from about 0.010% to about 0.030% to prevent a coarse precipitate from being formed, and the coil spring steel is molded into a product that has a fatigue life of about 850,000 cycles or greater and a corrosion fatigue life of about 550,000 cycles or greater. 